Conventionally, there is well known an art of a hydraulic stepless transmission (HST) comprising first and second rotation shafts, first and second plungers reciprocated along the axis, first and second spools reciprocated along the axis, a cylinder block housing the first and second plungers and the first and second spools and rotated integrally with the first rotation shaft, a movable swash plate touching the first plunger at its swash plate surface whose slant angle about the axis is variable, and a fixed swash plate rotated integrally with the second rotation shaft while touching the second plunger at its swash plate surface whose slant angle about the axis is fixed characterized in that the slant angle of the movable swash plate about the axis is variable.
With regard to the hydraulic stepless transmission, a hydraulic servo mechanism is used as a driving mechanism regulating slant angle of the movable swash plate of a hydraulic pump or a hydraulic motor of variable capacity type.
The hydraulic servo mechanism makes large driving power from small operating power and regulates operating amount easily and accurately, thereby leading to wide use as a driving regulation means of a movable part of an apparatus.
The hydraulic servo mechanism conventionally adopted to the hydraulic stepless transmission is a double barrel type hydraulic servo mechanism generally comprising a piston slid into a cylinder so as to generate large driving power as an actual output and a servo spool built in the piston as an operation body for position control. The double barrel type hydraulic servo mechanism is excessively large against driving power generated thereby so as to reduce degree of freedom of layout.
With regard to the known art, in the case of using the hydraulic servo mechanism as a driving mechanism of the movable swash plate of the hydraulic stepless transmission, the hydraulic servo mechanism is separated from the hydraulic stepless transmission and is disposed separately in the vicinity of the hydraulic stepless transmission, and driving power is transmitted through a link mechanism. Then, the additional attachment space is required.
According to the above conditions, it is difficult to construct a hydraulic stepless transmission compactly.
With regard to the conventional art, a cylinder block adopted to the hydraulic stepless transmission is manufactured by forming a plurality of holes in a cylindrical steel material by mechanical processing and requires troublesome processes such as boring so that number of processes is many. Then, it is difficult to make the manufacturing easy.
The mechanical processing is required for details of the cylinder block so it is difficult to miniaturize the cylinder block, whereby it is also difficult to miniaturize the hydraulic stepless transmission.
Conventionally, each of the first and second spools is always slid reciprocally with fixed timing at the time of operation of the transmission so as to switch oil passages. If the timing is not uniform, the hydraulic stepless transmission is not actuated normally. Then, the timing must be uniform.
An art is required for reducing frictional resistance of sliding of the spools as much as possible so as to make the sliding of the spools smooth and to prevent any trouble such as seizing.